1. Field of the Invention
The present invention relates to an electrophotographic machine, and more particularly to an electrophotographic machine wherein an original image is projected on a photosensitive medium at a predetermined magnification ratio so that a magnified or reduced copy is obtained.
2. Description of Related Art
Generally, in an electrophotographic machine having an optical system employing a uni-focus lens as a projection lens, it is necessary for a change in magnification not only to more the lens to a certain position to adjust the magnification in the direction perpendicular to the scanning direction (widthwise direction) but also to move the reflection member (mirror) to a certain position in accordance with the conjugate length. Conventionally, a stepping motor has been usually user as a common drive power source for the above-described movements of the lens and the reflection member, and a cam has been used for adjusting the travels of the lens and the reflection member. In more detail, a stepping motor is rotated at a certain angle to move the lens according to a magnification ratio, and a cam linked to a rotary force transmission mechanism provided for the stepping motor moves the reflection member.
In practice, uni-focus lenses even in the same production line have their respective own characteristics due to errors in assembly and vary in focal length in a certain range. Since the lens is directly driven by the stepping motor, it is possible to control the movement of the lens in accordance with its focal length. In order to control the movement of the reflection member in accordance with the focal length o the lens, it is necessary to design an prepare a cam having a special configuration corresponding with the focal length of the lens. However, cams for a gel are produce only in a limited number of kinds in order to avoid any increase in production cost attributed to possible complication of control data an increase of parts in kin. Hence, in reality, it is impossible to control the movement of the reflection member in accordance with the focal length of the lens. When a cam an a cam follower are used, a significant amount of toque loss will take place, and the load on the motor will increase.
There is another problem in changing magnification is that the mutual adjustment between focus an magnification is complicated. When it is detected that the lens an the reflection member are set well as regards to focus but poorly as regards to magnification, the lens should be moved, which movement causes a movement of the reflection member, and the focus will be lost. It is therefore necessary to repeat the above-mentioned operation until correct focus and magnification are obtained, which is a complicated procedure.
To eliminate the above problems, there is a scheme of driving the projection lens and the reflection member independently with separate stepping motors. In this case, however, since the smallest unit of the travels of the projection lens and the reflection member is one step of the respective motors, there will be errors in moving the lens and the reflection memberby at most a half of the amount corresponding to one step of the respective motors. The above-mentioned errors deteriorate the optical accuracy in focus and magnification.